On December 11, 1863, Federal batteries on Morris Island fired eight rounds at Charleston starting at 1 a.m. Confederate batteries on James Island responded in kind. But by 6 a.m. all was quiet again. The echos of the morning’s action faded with the dawn. With the Second Major Bombardment of Fort Sumter now over, the combatants were staring at each other instead of firing cannons.

The Confederates in the fort took the respite to repair and improve the fort. The garrison picked up the routine of the posting – watching, guarding, resting, and drawing rations. At around 9:30 a.m. the routine stopped with a bang. An explosion followed by smoke from the southwest corner of the fort now held the attention of all observers around the harbor. So what exploded?

During the long bombardment, Confederate engineers rearranged the magazines within Fort Sumter. The Three Gun Battery had its own service magazine within the casemate walls. Another magazine for the heavy guns took up a bombproof in the center of the fort. And a third magazine, utilizing the one remaining original magazine inside the gorge face, was near southwest corner of the fort. Captain John Johnson described that arrangement in The Defense of Charleston Harbor:

This was the lower pair of chambers, those belonging to the upper casemates having been long since abandoned and partially filled with sand. The small-arms magazine, then, as it was called to distinguish it from the others, lay secure behind the massive protection of brick and stone, together with the fallen débris afforded by the exterior of the gorge in that vicinity. It was next to the spiral stair-tower in the south-western angle, and adjoining the western or city face of the fort. But both chambers were not in use for purposes of a magazine: the limited accommodations of the post furnished no secure place for the commissary stores, and they were accordingly kept in the outer chamber of this magazine. It was know to be hazardous so to divide the use of the two chambers, but there seemed to be a necessity for it in the straitened circumstances of the fort. The contents of the inner chamber consisted of an incongruous assortment of rifle-cartridges, fixed ammunition for howitzers, hand-grenades, shells and torpedoes, etc. etc. Perhaps three hundred pounds of powder would have made up the explosive total.

The green circle is the location (generally) of the staircase tower. The red shaded area is the inner magazine where ammunition and powder was stored. The yellow is the portion used for commissary stores. Here’s what that corner of the fort looks like today:

The arched passage between the two chambers of the magazine is just right of center. The low wall in the center, with an inset section, separated the kitchen space from the inner chamber of the magazine. From this view, the explosion took place somewhere around that arched passageway.

The Confederates never ascertained exactly what caused the explosion. Federal artillery was quickly dismissed, as no shots were observed. Because of the morning ration issue, many persons were in the confined passage way. Some proximate causes suggested include pipe tobacco ash igniting stray powder, candles setting off a barrel of whiskey, or mishandling of stores. Regardless of the cause, the resultant explosion started a conflagration.

The materials in those rooms were immediately ignited, their occupants killed, and those stationed in the adjoining passages either killed, or burned with greater or less severity.

The passages leading to the lower and upper tiers of casemates, and those casemates themselves, were filled instantly with the most dense smoke, introduced by a blast of great strength, whose flame was visible from the room occupied as headquarters. In total darkness, the occupants rushed from the stifling smoke to the open embrasures, leaving their arms and blankets behind. The continuance of the smoke prevented any prolonged attempt to obstruct the progress of the fire.

On Sullivan’s Island, observers first feared the cotton used to fill empty casemates had caught fire. The fire took out the telegraph station on the southwest corner of the fort. So the exact nature of the disaster was not apparent for several hours.

During that time, the Federals noticed the smoke and realized something was amiss in Fort Sumter. The batteries on Morris Island came to life and fired on Fort Sumter 220 times, by Confederate accounts. The short resumption of fires, from the Parrotts, columbiads, and mortars was later designated “the second minor bombardment” of Fort Sumter. Confederate mortars on Sullivan’s Island fired counter-battery in an effort to stop the Federal gunners. The Federal artillery hindered the fire-fighting and wounded Elliot. But at dusk the firing tapered off. (Johnson would complain about this bombardment after the war, drawing a comparison to General P.G.T. Beauregard lifting fires on Fort Sumter on April 13, 1861 when fires broke out among the wooden buildings.)

Almost twelve hours after the explosion, details finally arrived at department headquarters. Colonel George Harrison dispatched 100 men to bolster the fort’s garrison. Fort Johnson was to make ready hand-grenades, howitzer ammunition, musket cartridges, and 200 muskets to replace those lost by the garrison in the fire. In addition, headquarters requested to “have a fire engine ready to go to Fort Sumter.” To replace the lost communication equipment, torches and turpentine, a replacement telegraph set, and 200 yards of cable, along with pencil and paper for the operator, went to Fort Sumter.

By morning the fires were under control. But with everything combustible burnt or burning, the temperatures in the casemates remained high. Fires smoldered on until December 18. Only after ten days did the bricks cool sufficiently to permit soldiers to re-enter the magazine to inspect. With all wooden parts of the fort consumed, the casemate structures were questionable at best. The roof of the magazine had collapsed. Johnson and the other engineers immediately set to work repairing, rebuilding, and reorganizing the fort’s defensive works.

The human cost stood in contrast with the earlier bombardments of Fort Sumter. Up to December 11th, through all the bombardments of the summer and fall, the fort’s garrison had only suffered 156 casualties total. Elliot recorded eleven killed and forty-one wounded on December 11, both from the fire and Federal bombardment. By that measure, the accident was much worse than the Federal shot and shell.

On the other hand, the physical damage to the fort was easier to mend. Johnson and the other engineers assigned that duty had become very good at manipulating rubble so as to make a better earthwork.

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